.. _sync_requirements:

Synchronization Requirements
----------------------------------------------

* The shared buffer problem (see :ref:`race_condition`) and most
  synchronization problems require :term:`mutual exclusion`: Only one process
  or thread can be in the :term:`critical section` at a time.

* Other problems, such as :ref:`readers_writers` allow more than one read only
  thread in the :term:`critical section` at the same time. 
  (See :ref:`classic_problems`)

* Even to implement a simple lock, requires :term:`mutual exclusion`::

    shared boolean locked;
    ...

    while( locked ) WAIT();
    /* THIS IS A BAD TIME FOR A CONTEXT SWITCH  */
    locked = True;
    /**********************/
       Critical Section
    /**********************/
    locked = False;
    

* Without :term:`mutual exclusion`, results of multiple execution are not
  determinate.

* Need an **Operating System provided mechanism**.